Thanks Rachel Baker for this :)

custom-endpoint.js

register_json_route( 'wp', '/comments/(?P<id>[\d]+)', array(
        array(
            'methods'  => WP_JSON_Server::READABLE,
            'callback' => array( $controller, 'get_item' ),
            'args'     => array(
                'context'  => array(
                    'default'  => 'view',
                ),
            ),
        ),
        array(
            'methods'  => WP_JSON_Server::EDITABLE,
            'callback' => array( $controller, 'update_item' ),
            'args'     => array(
                'post_id'      => array(),
                'status'       => array(),
                'content'      => array(),
                'author'       => array(),
                'author_email' => array(),
                'author_url'   => array(),
                'date'         => array(),
            ),
        ),
        array(
            'methods'  => WP_JSON_Server::DELETABLE,
            'callback' => array( $controller, 'delete_item' ),
            'args'     => array(
                'force'    => array(),
            ),
        ),
    ) );

This is a WordPress endpoint, but plugins and themes can register their own, replacing "wp" (which is reserved for core) with something like "plugin/events/v1". And then of course custom arguments for the endpoint.

Also from the team's notes:
Apart from the core routes, routes should be prefixed to avoid overlapping between plugins. The version should be included with the namespace to create a route prefix. The easiest way of achieving this is to require both a namespace and version when defining the routes.